1. Field of the Invention
The knee joint prosthesis with mobile congruent insert subject of the present invention allows the pivoting rotation of the insert, this being inserted into a cylindrical tapping of the plastic piece.
The knee prosthesis is an implant destined to replace a deteriorated knee joint. The knee prosthesis (of the three-compartment type) is comprised of one femoral, one femoral, and one patellar component. The femoral component is made of metallic material and generally reproduces the condyles anatomic curvatures. The tibial component is generally comprised of a metallic tibial tray, a shaft and a spacer or an insert of plastic material (polyethylene) which forms the contact surface.
At the present, the main problem of this kind of prosthesis is that it gets loose, which requires its replacement with a new one by surgical operation.
Said loosening is due on one hand to the high stresses being transmitted in the interphase bonexe2x80x94implant, and on the other hand to the particles arising from the wear that causes the decay of the bone to which the prosthesis is attached.
The designs of knee prosthesis with mobile polyethylene insert therefore have the aim to reduce the forces in the interphase between the tibial component and the bone, and the decreasing of the polyethylene""s wear by using contact surfaces with similar curvatures (congruencies) therefore achieving smaller contact pressures.
From the knee prostheses of this kind known in the art mention is to be made of those which have a tibial tray over which two plastic pieces drive independently, these plastic pieces generally sliding on curve-sided guides built in the tibial tray and being indicated for cases of conservation of the cruciate ligaments, or of the posterior cruciate ligament only.
The main inconvenience of prostheses having two independent mobile inserts is that they may turn up luxation problems as there exists the possibility that the plastic pieces might come out of the metallic or tibial piece, because they have no artificial bumpers to set up a limit to the movement, but they rather directly depend on the environment.
Another inconvenience of these prostheses is that the relative movement of some pieces with respect to the others is carried out by means of guides, the surgical technique therefore being very complicated, as any small failure which displaces any of the two parts of the guide causes coupling and performance problems during their work.
Another type of prostheses are those ones that have a rotating one-piece plastic insert, in case that both ligaments are sacrificed. The surfaces are completely congruent in extension in both configurations, there being a variant of this system, and being this one a prosthesis with a sliding plate, providing one grade more of liberty.
With this type of inserts there exist punctual contact areas between the insert and the rotation center or tray, said last point depending on the sliding system between the tray and the insert.
Another type of prosthesis exhibits a total congruence in extension by means of cylindrical surfaces, and the sliding of the insert with the platen is achieved by means of the conical elliptical shape of the base of the insert in a housing wrought in the tibial component, in such way that at the same time the insert rotates, the cone of the insert also rotates on its housing.
In these cases there also may appear zones of punctual contact between the cone of the insert and its metallic housing in the varusxe2x80x94valgus movements that are produced in the knee joint during the flexionxe2x80x94extension motion.
There also exists another knee prosthesis comprising a rotating one-piece insert of polyethylene showing a total congruence in extension and flexion, wherein the contact in extension is located in the zone of central support, while as the articulation is flexed, some lateral bands of the polyethylene insert with a radius similar to that of the femoral condyles in their posterior part come in contact.
These inserts with one sole pivotal center and total congruence in flexion and extension also require a very precise surgical technique, as they don""t have any possibility of anterior posterior displacement.
All above mentioned inconveniences may be avoided by using the knee prosthesis with mobile congruent insert object of the present invention. In this prosthesis the surface of the plastic insert reproduces in its superior part of the geometry of the two femoral condyles, therewith, in comparison to other designs, the contact surface between both is largely increased (definition of the term congruence) what avoids punctual contact tensions that provoke the debilitation of the bone the prostheses are attached to, also lowering the effect of the existing shear stresses between the interphase insertxe2x80x94femoral component, avoiding the dislocation between both. In its completely planar lower part the insert has a tapping wherein is housed a pivot provided on the tibial tray.
On the contrary, the tibial part is composed of a platen and a keel-shaped shaft (cross-section) that avoids rotation of this component on the tibial axis once it has been implanted. In its superior part there is provided a cylindrical pivot located in the central middle third.
Over the surfaces of the plastic insert the surfaces of the condyles of the femoral component slide reproducing the flexion motion of a human knee. Also, the tapping allocated in the lower part of the insert fits into the cylindrical pivot of the tibial platen allowing a relative movement of the whole assembly, femoral component and insert, on the tibial component.
The advantages of the proposed invention are:
Congruence of the contact surfaces between the plastic component and the femoral component, total in extension and partial in flexion, leading this to a reduction of contact stresses and therefore of wear, this way lengthening the useful life of the implant and also considerably reducing its discomforts.
Light allowance of anteriorxe2x80x94posterior movement thanks to the geometry existing between the femoral component and the plastic component and the rotary motion supported by the cylindrical tapping in the lower part of the insert through which the pivot of the metallic tray is introduced. Anteriorxe2x80x94posterior displacement range that allows to correct light alignment failures of the articulation during the surgical operation, requiring a less rigorous technique and therefore facilitating the intervention, thereby implicating less risks for the patient.
This insert does not bear any guides that steer its relative movement with respect to the tibial tray, so, besides having a greater ability of movement And facilitating the surgical technique, this insert is freed from unnecessary and damaging stresses (sheer stresses and punctual tensions).
In order to facilitate a better comprehension of this disclosure and forming an integral part of it, shall be shown hereinafter a series of illustrating but not limiting drawings in which there is depicted the following: